It is known that photographic elements which contain unhardened gelatin layers, containing silver halide emulsions, can be image-wise made insoluble upon exposure to light and processing in the presence of tanning developers. In fact, the oxidation products of the tanning developer formed in the exposed regions of the image have the property of diffusing through the gelatin layers containing them and of hardening the gelatin itself. The unhardened regions, being unexposed and undeveloped, can be washed away with water or can be totally or partially transferred onto an image-receiving material.
U.S. Pat. No. 3,364,024 describes photographic elements consisting of a base having coated thereon a gelatin silver halide sensitive emulsion layer and having in adjacent position thereto to pigmented unsensitive layer of unhardened gelatin. Upon development of the exposed layer in the presence of a diffusing tanning developer of the hydroquinone type, the oxidation products of the hydroquinone upon diffusion harden the exposed regions of the sensitive layer and those regions of the pigmented layer adjacent thereto.
To make the process work properly, it is essential that the exposed silver halide be reactively associated with unhardened gelatin during development, where the developing agent is oxidized by the exposed silver halide to react with the (unhardened) gelatin to harden it. The silver halide emulsion and the unhardened gelatin can be present in the same layer or in different layers which are part of the same element, or in different elements which are put into contact prior to development. In any case, during development, it is essential for them to be in reactive association (with no barrier therebetween), to give the oxidized developer the possibility of diffusing from the reacted silver halide grains (reduced to silver) to the (unhardened) gelatin. The diffusing tanning developer can be comprised either in the layer containing the emulsion and/or the unhardened gelatin or in the developing bath.
U.S. Pat. No. 4,369,245 describes particular values of the silver coverage and silver/gelatin ratio of the sensitive layer and of the gelatin/tanning developer ratio in a photographic element for tanning development, which allow to obtain a better image quality and a higher exposure latitude.
In practice, for reasons of producivity, it is preferred to process said elements in an automatic processor provided with transporting rollers for the elements. Said processors schematically consist of a first tank containing an activating bath which consists of an alkali water solution and subsequent tanks containing a stop bath at neutral or acid pH and a washing bath by means of water sprays. The processors are provided with a series of rollers, of which some are dipped in the solutions and other are partially or totally outside them, to transport the elements into the tanks and from one tank to another.
The processing of photographic elements for tanning development performed in an automatic processor under such conditions, with respect to the manual processing, causes a decrease of the image quality even in elements whose critical parameters have been optimized, and this decrease appears as a worsening in the sharpness of the type edges and as fog formation.